guix: Ingesting TeX Live Subversion checkouts

Hello!

As discussed with @anlambert at the SWH community workshop back in February, source code of the 4k+ TeX Live packages in Guix cannot currently be recovered due to its peculiar arrangement: the source code of those packages is obtained by checking out individual directories and then combining them, as in this example.

IIRC, @anlambert suggested extending sources.json to include the list of sub-directories to be checked out; the loader would then compute a nar-sha256 ExtID for the combined directories.

Right now, this particular example translates to:

    {
      "type": "svn",
      "svn_url": "svn://www.tug.org/texlive/tags/texlive-2023.0/Master/texmf-dist/",
      "integrity": "sha256-jLwRV6qTQv1ddjb14mnOuwHPTEQ77/J7UM/auqVI++8=",
      "outputHashAlgo": "sha256",
      "outputHashMode": "recursive",
      "svn_revision": 66594
    },

Any preference on how to include information about sub-directories? Maybe something like:

  "svn_subDirectories" = [ "doc/generic/dehyph-exptl/", "doc/generic/elhyphen", ...]

WDYT?

Cc: @zimoun @samplet